The present invention relates to a light-emitting gallium nitride-based III-V group compound semiconductor device with high light extraction efficiency, especially to a compound semiconductor device that reduces the operating voltage and improves the efficiency of light extracting.
Refer to FIG. 1, the structure of conventional light-emitting gallium nitride-based III-V group compound semiconductor device is disclosed. The device includes a sapphire substrate 10′, a gallium nitride buffer layer 20′, a n-type gallium nitride contact layer 30′, an indium gallium nitride (InGaN) emitting layer 40′, a p-type gallium nitride layer 50′, a p-type gallium nitride contact layer 60′ (from layer 20′ to layer 60′ are called epitaxy structure)′, a transparent conductive layer 70′ made of Ni/Au. Moreover, a p-type metal electrode 80′ is stacked over the transparent conductive layer 70′ and a n-type metal electrode 90′ is arranged over the n-type gallium nitride contact layer 30′.
In order to increase the light extraction efficiency of the device, the optical transparency of the transparent conductive layer is increased or a reflective layer is added under the light-emitting layer. However, both methods can only increase the light extraction efficiency of the emitting light along vertical direction.
The refractive index of the epitaxy structure of gallium nitride is 2.4, the refractive index of the sapphire substrate is 1.77, and the refractive index of the packaging resin is 1.5. Due to the waveguide effect, part of the light emitting from the light-emitting layer is reflected by the sapphire substrate and the packaging resin and then is absorbed by the multi-layer epitaxy structure of gallium nitride. Thus the light extraction efficiency is decreased. In order to break the waveguide effect, a texturing surface or a rough surface is provided on the surface of the light-emitting compound semiconductor device so as to reduce reflection of light through various interfaces with different refractive indices.
During the growth of the epitaxy, the texturing or rough surface are formed artificially, please refer to the Taiwanese patent application No. 092132987, the process has been described therein. Moreover, in order to increase the light extraction efficiency and reduce the operating voltage, a structure is disclosed in the Taiwanese patent application No. 093204255 whose applicant is the same with the present invention. A conventional transparent conductive layer made of Ni/Au is replaced by a transparent conductive window layer whose optical transmittance is better than Ni/Au and having good ohmic contact with the texturing surface layer so as to reduce the operating voltage.
Furthermore, if the bottom of the multi-layer epitaxy structure of gallium nitride is also provided with a texturing surface or a rough surface, the light extraction efficiency of the device is further improved.